Introduction
With the development of vitrification technology, the survival and
intact cell rates are increasing and the implantation, clinical
pregnancy and live birth rates of the vitrification are similar to the
fresh.1-3 Embryo cryopreservation makes it possible to
conserve surplus embryos for further use, and increases cumulative
live-birth rates with one oocyte retrieval operation. The advantages of
frozen-thawed embryo transfer (FET) are not only decreasing the rate of
ovarian hyperstimulation syndrome (OHSS), but also providing a more
favorable intrauterine environment for embryo implantation and
placentation by avoiding the supraphysiologic condition that occurs
after ovarian stimulation.4-7 Hence, the proportion of
FET is increasing in assisted reproductive technologies. The factors
affecting the outcome of FET are female age, embryo characteristics,
endometrial preparation protocol, number of embryos transferred and so
on.8, 9
Currently, the two most common procedures to select embryos for FET
differ in the duration of the post-thawed culture: one relies upon the
observation of blastomere survival after thawing, requiring a short
culture; another is dependent on the observation of blastomere
proliferation, requiring a long culture, generally overnight. Some
studies indicated that cleavage stage embryos with
low
blastomere number had significantly decreased developmental potential
and pregnancy rate,10-12 and extended culture of these
embryos to blastocysts can resulted in a similar clinical pregnancy rate
as that of blastocysts derived from embryos with ≥ 7
blastomeres.12, 13 The influence of post-thawed
culture for about 18 h on the outcome of FET is still
controversial,12, 14, 15 and the reasons may be
related to the different sample sizes, inclusion criteria, embryo
freezing methods, embryo quality and age distribution of patients in
different studies. Previous studies focused on the comparison between
short culture and long culture, while ignoring the effect of age and
embryo quality on implantation and live birth rates. In this study, a
retrospective analysis improved sample size (containing 9832 FET
cycles), age distribution (divided into three groups: <35,
35-39 and > 39) and embryo quality (good-quality embryos)
to evaluate the influence of long post-thaw culture period on pregnancy
and clinical outcomes.